The OED uses frogs and ravens as the only exemplars of beings which make this sound in the first definition of “croak the noun” and “croak, the verb”.  Intrigued, I read onward.  Obsolete second meaning includes “forbode evil (like the raven)”, without note or explanation about this anthropomorphization, just one example to make the hair rise:

1609   Shakespeare Troilus & Cressida v. ii. 193   Would I could meete that roague Diomed I would croke like a Rauen, I would bode, I would bode.

Goblins’ singing is so laughable as to be called croaking, but crow and bird croaks are eerie – and perhaps the sounds of Roäc are truly ominous!

• 04.018 The goblins began to sing, or croak,
• 04.036 croaking, jibbering and jabbering;
• 11.008 and again the harsh croak of a bird.
• 11.011 followed ever by croaking crows above them,
• 15.014 he croaked
• 15.022 croaked Roäc,

“croak, v.” OED Online. Oxford University Press, March 2015. Web. 27 May 2015.